Synchrotron radiators have been used for over 30 years to produce intense, collimated x rays for scientific and some minor industrial applications. Such applications have been limited since synchrotron radiators are expensive and large, and require large amounts of power to operate and higher electron-beam energies to produce even soft x rays.
In the prior art, transition, parametric and channeling radiators have been proposed as source of collimated radiation, since these sources are less expensive and require lower electron-beam energies. In U.S. Pat. No. 5,077,774, "X-ray Lithography Source" Piestrup, Boyers and Pincus demonstrated that transition radiation can be used as a source of soft x rays for lithography. X rays are produced in a collimated annular cone when relativistic electrons are passed through multiple thin foils. In that patent it was shown that the electron energies greater than 17 MeV could be used to generate soft x-rays in the 800 to 2000 eV range which are ideal for x-ray lithography for the fabrication of integrated circuits. In that patent, electrons are produced from an accelerator and pass once through the transition-radiator foil stack. The electrons are then "dumped" into an appropriate absorber (called a beam dump) where they are no longer used and spurious and harmful radiation is minimized. These radiators could be excellent sources of x rays provided high enough electron-beam current could be sent through them. However, given the modest currents available from Linear Accelerators (LINAC's), Roger Carr has shown that in most cases periodic-medium radiators driven by such conventional LINACs are not practical (Roger Carr, Nuc. Instrum. Meth. vol. 347, p. 510 (1994). As pointed out by M. A. Piestrup, D. G. Boyers, C. I. Pincus, J. L. Harris, X. K. Maruyama, H. S. Caplan, R. M. Silzer, D. M. Skopik, "Beryllium-foil transition radiation source for x-ray lithography," Appl. Phys. Lett. vol. 59, pp. 189-191, 1991., to make transition radiation a competitive source of soft x rays for lithography, one needs to increase the x-ray flux to power levels of 10 mW/cm.sup.2 at the mask-wafer surfaces. This is a factor of one tenth of what was observed in that cited patent.
The cost of a LINAC ($1m to $5m) to drive a parametric x-ray (PXR) source would make a radiographic system prohibitively expensive when compared to conventional imaging systems (price range of mammography systems: $75K to $150K). Because of its narrow bandwidth and directionality, PXR could provide a substantial improvement in image quality over that of a conventional system; however, the higher cost would be difficult to justified.
In the prior art, Betatrons have been long known as relatively inexpensive sources of electrons and hard x-ray emission when compared to LINACs and storage rings. The betatron is technically a simple device, robust and easy to fabricate when compared to LINACs. The principle of betatron acceleration is based on Faraday's Law: an alternating magnetic field is surrounded by a rotational electric field. Electrons can be accelerated by this electric field, while at the same time being guided by the magnetic field. The process involves injecting low-energy electrons into an evacuated toroid and then increasing the fields that "link" the toroid. As the magnetic fields rise, the electrons are accelerated by the induced electric field and the process is allowed to continue until the electrons acquire the desired kinetic energy. The first betatrons were successfully designed and operated by Kerst in the 1940's (D. W. Kerst and R Serber, Phys. Rev. 60, 53 (1941)).
Once the electrons reach the desired energy, they can be driven into a stationary tungsten target placed within the betatron toroid, thereby producing bremsstrahlung x-rays. These internal bremsstrahlung radiators have been used since the beginning of betatron development. In general, these have been thick tungsten radiators for the production of very hard x-rays. The electron beam is stopped or drastically scattered so that only one pass of the electrons is achieved.
Betatrons were in use throughout the 1960's and early 1970's for the purpose of cancer therapy. Manufacturers included Allis-Chalmers, Varian Associates and Brown-Boveri. The betatron accelerator has many advantages for generating high-energy electron beams compared with other acceleration techniques. At energies below 35 MeV, betatrons have a much simpler construction than either linear induction or RF LINACs, and as such are inherently more reliable. Moreover, the output beam energy can be easily varied by either changing the maximum induction field in a given acceleration cycle, or keeping the maximum induction field fixed, varying the time at which the beam is driven into the target. Unlike LINACs, increasing the betatron's beam current does not cause the output energy to fall. On the other hand, the standard iron-core betatron is current-limited compared with the LINACs of similar size and energy. Space-charge effects limit the amount of charge captured per acceleration cycle (D. W. Kerst, Phys. Rev. 60, 47 (1941)). When this is coupled with a low frequency of operation (which is limited by ferromagnetic core loss), time averaged currents of only a few tenths of a .mu.A are achieved. Today, there is no commercial viability for such a dim source and, currently, only Russian Betatrons are being constructed.
In a German patent #0-276-437 by inventors Wolfgang Knupfer, Manfred Pfeller, and Max Huber it was proposed, but not tested, that crystals be precisely aligned in electron storage rings to produce x-ray by the channeling of the electrons (or positrons) through the planes of the crystal. Transition, parametric, bremstrahlung radiators are not mentioned. In one embodiment, a storage ring has its electrons supplied by a microtron and a small RF accelerator structure in the storage ring is utilized to supply lost energy to the recycling electrons. The issue of the necessity of how to maintain the electron beam in the storage ring was not discussed. Indeed, no correct estimates on the number of passes through the radiator were given. A claim of achieving beam times "on the order of an hour" are not possible with internal solid radiator placed inside of the ring. As was shown in the parent application (Piestrup, Lombardo, Kaplin, and Skopik, "Thin Radiators in Recycled Electron Beam" in U.S. patent application Ser. No. 08/872,636 filed Jun. 10, 1997), total beamtime is limited to a few microseconds in accelerators of even high energy, 800 MeV.
The German patent does not state how the electron beam is to be injected into the storage ring using the microtron. In all schemes for storage rings and synchrotrons the microtron or LINAC can only inject for one pass of the electron beam. The method of injecting the beam into the ring requires an electrostatic field or magnetic field by a "kicker" magnet to inject the electrons. Returning electrons that meet this field after one pass will be ejected out of the ring. In all synchrotron and storage rings, the kicker magnet is turned off after one pass of the beam and the beam is allowed to "dampen" into a stable orbit which is smaller than the injection orbit. Note that this would also limit the length of the injected electron beam pulse and, hence, for small rings such as the one proposed in the German patent, this effectively severely limits the average current.
In the parent application of this case, it was suggested and demonstrated that storage rings can be utilized to increase the average current through other thin solid radiators (besides channeling crystals). The thin radiator thickness is chosen to be small enough such that the electrons pass through, yet thick enough so that sufficient x rays are produced. Thus, the radiators can seen to be quasi-transparent to electrons. In these schemes a thin radiator is placed in the storage ring, synchrotrons or cyclical accelerator where the electron beam passes through the thin radiator many times. Thus the average current through the radiator is dramatically increased. Since the x-ray production through the thin radiator is directly proportional to the electron current or the number of electrons passing through the radiator, the x-ray flux increases proportionally to the average current and the overall efficiency of the radiator. As in the case of synchrotron radiators, such storage rings are very expensive and the amount of x-ray flux is lower than that achieved by using the ring for the generation of synchrotron emission. Unlike the German patent cyclical accelerators are also proposed in the parent patent application as one method of achieving multiple passes through the radiator.
In a proof of principal experiment of the parent application by Piestrup et al., electrons are injected into the storage ring at the rate at which the accelerator is being pulsed. The pulse length is short enough that the electrons only make one pass around the ring during the injection pulse. Thus, there is no problem of the electrons seeing the electrostatic field of the injector. This limits the length of the pulse and requires that large rings be used.
Prior to the parent application by Piestrup et al., it was assumed that even a thin solid target inside a storage ring would result in sufficient scattering and energy loss that the electrons would only make one pass through the target. However, subsequent analysis and experiments show that recirculated electron beams can achieve multiple passes through radiators that have sufficient thickness for efficient x-ray emission. In the proof of principal experiment carried out at the Saskatchewan Accelerator Laboratory (SAL), radiators were thin 0.18 to 9 .mu.m and the electrons were of sufficient energy 118 to 252 MeV so that the electrons passed through the radiators many times. A variety of single and multiple foil transition radiators made of different foils were utilized (C, Al, Cu and Ta). Passes of between 5 and 385 passes were observed depending upon the electron-beam energy and the radiator thickness and density. In this energy range (118 to 252 MeV) and radiator thicknesses, the Al radiators showed the number of passes were proportional to the radiator thickness. Thus the measured power from the single-foil transition radiator was the same as the measured power from the 9-foil transition radiator.
In the proof of principal experiment carried out at the Tomsk Sirius storage ring for the parent application, both parametric and transition radiators were utilized. A thin radiator of 48 .mu.m Silicon was placed in the storage ring and 20 passes were measured for a 800 MeV electron beam. Photon energy of 20 keV was generated from this crystal. Note for this ordinary storage ring, the number of passes is small. Thus, it was not at all obvious from the German patent, that enough passes could be achieved using the microtron driven storage ring scheme that was briefly discussed therein. Indeed, the German patent makes no references to the number of passes that could be obtained.
These proof-of-principal experiments were also presented for high energy electron beams (E&gt;800 MeV) in M. Yu. Andreyashkin, V. V. Kaplin, M. A. Piestrup, S. R. Uglov, V. N. Zabaev, "Increased X-ray Production by Multiple Passes of Electrons trough Periodic and Crystalline Targets Mounted Inside a Synchrotron," Appl. Phys. Letts. 72 pp. 1385-1387 (1998) and at moderate energy electron beams (E&gt;118 MeV) M. A. Piestrup, L. W. Lombardo, J. T. Cremer, G. A. Retzlaff, R. M. Silzer, D. M. Skopik and V. V. Kaplin, "Increased x-ray production efficiency from transition radiators utilizing a multiple-pass electron beam" The Review of Scientific Instruments 69, No. 6, pp. 2223-2229(1998).
In the parent application, the number of passes was small especially when the electron beam energy was at it lowest (measured at 118 MeV). This was most likely due to the long path length for one trip around the storage ring and the finite aperture of the beam pipe and various magnet gaps around the ring. The latter were limited in size to maximize the magnetic field. These small aperture diameters, d, functioned with the long electron path length, L, to form a large aspect ratio (L/d). Thus if the electrons deviated from their path through the ring, they easily collided with the walls of the beam pipe of the magnet gaps. It might be possible to design a storage ring with a smaller aspect ratio; however, this would be expensive and require larger magnets and beam pipe.